1. Field of the Invention
This invention generally relates to ion-exchange materials and, more particularly, to ion-exchange materials that are useful as electrolytes for electrochemical fuel cells.
2. Description of the Related Art
Ion-exchange materials are useful over a broad range of applications, and may generally be categorized as either anion- or cation-exchange materials. Such materials have been used in fields such as chromatography, catalysis, electrochemical processes, the creation of super acids and super bases, and for the separation, concentration and/or purification of ionic species. One important application of ion-exchange materials is their use as electrolytes in electrochemical fuel cells. In this application, the electrolyte commonly conducts protons and thus may be characterized as a cation-exchange material. Such cation-exchange materials typically constitute an organic polymer having acidic functional groups attached thereto. The acidic functional groups, in turn, comprise corresponding cations, which, in the context of fuel cell electrolytes, are more commonly protons. When the electrolyte is in the form of a membrane, the ion-exchange material is often referred to as a proton-exchange membrane or xe2x80x9cPEM,xe2x80x9d and fuel cells incorporating such a membrane are referred to as PEM fuel cells. Cation-exchange materials may also be incorporated into PEM fuel cells in other forms, for example, as components in the catalyst layers or as electrode coatings.
In general terms, an electrochemical fuel cell converts a fuel (such as hydrogen or methanol) and oxygen into electricity and water. Fundamental components of PEM fuel cells include two electrodesxe2x80x94the anode and cathodexe2x80x94separated by the PEM. Each electrode is coated on one side with a thin layer of catalyst, with the PEM being xe2x80x9csandwichedxe2x80x9d between the two electrodes and in contact with the catalyst layers. Alternatively, one or both sides of the PEM may be coated with a catalyst layer, and the catalyzed PEM is sandwiched between a pair of porous electrically conductive electrode substrates. The anode/PEM/cathode combination is referred to as a membrane electrode assembly or xe2x80x9cMEA.xe2x80x9d Hydrogen fuel dissociates into electrons and protons upon contact with the catalyst on the anode-side of the MEA. The protons migrate through the PEM, while the free electrons are conducted from the anode, in the form of usable electric current, through an external circuit to the cathode. Upon contact with the catalyst on the cathode-side of the MEA, oxygen, electrons from the external circuit, and protons that pass through the PEM combine to form water.
Desirable characteristics of a PEM include good mechanical properties, high conductivity, resistance to oxidative and thermal degradation, and dimensional stability upon hydration and dehydration. A variety of materials have been developed with these characteristics in mind, including perfluorinated sulfonic acid aliphatic polymers such as those described in U.S. Pat. Nos. 3,282,875 and 4,330,654. One example is a product sold by DuPont under the trade name Nafion(copyright). This material has been used effectively in PEM fuel cells due to its acceptable proton conductivity, as well as its mechanical and chemical characteristics. More specifically, Nafion is a polytetrafluoroethylene-based ionomer containing sulfonic acid groups to provide proton conductivity.
A variety of aromatic-based polymers for PEMs have also been investigated; such as films made from sulfonated poly-xcex1,xcex2,xcex2-trifluorostyrene. However, these membranes have unfavorable mechanical properties when wet and become brittle when dry (see Russian Chemical Reviews 59:583, 1988). Some improvement is realized when poly-xcex1,xcex2,xcex2-trifluorostyrene is blended with polyvinylidene fluoride and triethylphosphate plasticizer. However, the blended material is still not satisfactory for use as PEMs in fuel cells (Fuel Cell Handbook, A. J. Appleby, published by Van Nostrand Reinhold, p. 286, 1989).
Polymers with more favorable mechanical properties have been derived from copolymerization of xcex1,xcex2,xcex2-trifluorostyrene with a variety of substituted xcex1,xcex2,xcex2-trifluorostyrenes, as described by U.S. Pat. No. 5,422,411. Substituents in this context include alkyl, halogen, CyF2+1, OR (where R is alkyl, perfluoroalkyl, aryl), CFxe2x95x90CF2, CN, NO2, OH, and sulfonyl fluoride. Such copolymers may be prepared via an emulsion-free radical polymerization, followed most commonly by reacting with a sulfonating reagent to introduce sulfonic acid functional groups.
Enhanced mechanical strength and dimensional stability have been achieved when PEMs are prepared as composite materials. One approach is to first form the polymer into a membrane and then laminate the membrane to a porous substrate. Another approach is to impregnate the porous substrate with a solution of the polymer. In either case, the substrate imparts desired mechanical properties and dimensional stability. One example of this approach is the impregnation of Gore-tex(copyright) (a porous polytetrafluoroethylene) with Nafion as described in Journal of the Electrochemical Society, 132, 514-515, 1985. Also, U.S. Pat. No. 5,985,942 describes composite membranes prepared by impregnating a porous substrate with various substituted xcex1,xcex2,xcex2-trifluorostyrene polymers. Another approach to preparing PEMs having good mechanical properties, dimensional stability, and resistance to degradation is to graft functional components onto films having such characteristics. For example, U.S. Pat. No. 4,012,303 describes preparation of a PEM by grafting xcex1,xcex2,xcex2-trifluorostyrene onto a chemically resistant film, followed by sulfonation of the grafted material.
Development efforts directed to PEMs, such as those described above, have largely focused on improving mechanical properties, as well as oxidative and thermal stability. Less effort has been directed towards increasing the conductivity of such membranes, and that effort has been largely focused on the concentration or density and nature of ion-exchange groups incorporated into the polymeric material. Yet for fuel cell applications at least, it is very important that PEMs have high conductivity since commercial feasibility of fuel cells largely hinges on the power density achieved (i.e., electrical power output per unit of stack volume and weight).
Accordingly, there remains a need in the art for improved ion-exchange materials generally and, more particularly, for highly conductive ion-exchange materials useful as electrolytes in PEM fuel cells. The present invention fulfills these needs, and provides further related advantages.
In brief, this invention is directed to highly conductive ion-exchange materials that comprise a polymeric backbone and a plurality of pendent styrenic or fluorinated styrenic macromonomers covalently bonded thereto, wherein the plurality of pendent styrenic or fluorinated styrenic macromonomers comprise a uniform number of styrenic or fluorinated styrenic monomer repeat units, and wherein predominantly all of the styrenic or fluorinated styrenic monomer repeat units have at least one charged group. The charged group may be an acidic group or a basic group or a mixture thereof, or salts thereof.
The ion-exchange material may be formed by a copolymerization reaction between a backbone monomer and the plurality of styrenic or fluorinated styrenic macromonomers. In this case, reactive functionality on the terminal groups of the macromonomer will form part of the backbone, for example, through condensation or free radical polymerization of the macromonomers with other monomers. Further, depending on the nature of the terminal groups of the macromonomers, they may also form the entire backbone in the ion-exchange material, again through, for example, condensation or free radical polymerization reaction. Alternatively, the ion-exchange material may be formed by covalently bonding the plurality of styrenic or fluorinated styrenic macromonomers to an existing backbone, in the form of a bulk polymer or copolymer, comprising reactive sites. Furthermore, in some embodiments, the polymeric backbone may be a porous inorganic polymer, such as, for example, carbon (graphite) or TiO2, to which the macromonomer is grafted, or may be an electron-conducting polymer. Such materials have use, for example, as catalyst support materials.
A method is also disclosed for making a plurality of styrenic or fluorinated styrenic macromonomers having a uniform number of styrenic or fluorinated styrenic monomer repeat units. This method comprises the steps of (1) adding a free radical initiator into a mixture comprising one or more types of styrenic or fluorinated styrenic monomers, a solvent and a stable free radical agent, wherein the mixture is at a first temperature sufficient to form an adduct between a portion of the styrenic or fluorinated styrenic monomers, the free radical initiator and the stable free radical agent; (2) polymerizing the adduct with the styrenic or fluorinated styrenic monomers at a second temperature greater than the first temperature, wherein the second temperature is sufficient to sustain polymerization, and thereby forming the plurality of styrenic or fluorinated styrenic macromonomers having the uniform number of styrenic or fluorinated styrenic monomer repeat units; and (3) adding a reactive end-group to each of the plurality of styrenic or fluorinated styrenic macromonomers having the uniform number of styrenic or fluorinated styrenic monomer repeat units.
In a more specific embodiment, a proton-exchange membrane (PEM) is disclosed comprising an ion-exchange material having a polymeric backbone and a plurality of pendent styrenic or fluorinated styrenic macromonomers covalently bonded thereto, wherein the plurality of pendent styrenic or fluorinated styrenic macromonomers comprise a uniform number of styrenic or fluorinated styrenic monomer repeat units, and wherein predominantly all of the styrenic or fluorinated styrenic monomer repeat units have at least one acidic group. In further embodiments, a membrane electrode assembly (MEA) is disclosed comprising the PEM of this invention, as well as fuel cells containing such an MEA.
These and other aspects of this invention will be evident upon reference to the following detailed description and attached drawings. To this end, a number of articles and patent documents are cited hereinto to aid in understanding certain aspects of this invention. Such documents are hereby incorporated by reference in their entirety.